Diagnostic notes are not refereed

Postweaning multisystemic wasting syndrome (PMWS)
is a new disease first described in 1996.1,2 In the past 6-8,
months the number of affected herds has grown considerably, and there are
now confirmed cases in Alberta, Saskatchewan, Manitoba, Quebec, California,
Iowa, Indiana, and Spain. Furthermore, since 1994, a similar syndrome--"wasting
disease of piglets"--has been confirmed in as many as 200 herds in
France.

Because PMWS is a new disease, it is difficult to recognize and diagnose
(particularly, to differentiate it from porcine reproductive and respiratory
syndrome (PRRS) or postweaning anorexia/starvation). This paper reviews
the clinical and pathological characteristics of the disease to help the
veterinarian and pathologist reach a definitive diagnosis.

Clinical presentation

In our experience, PMWS is most commonly diagnosed in herds free of the
major enteric and respiratory diseases affecting swine. Such diseases include
Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae, progressive
atrophic rhinitis, salmonellosis, swine dysentery, transmissible gastroenteritis
virus (TGEV), and pseudorabies. Affected herds may be farrow-to-finish,
farrow-to-feeder pig, off-site nursery, or grower pig operations. The disease
has been diagnosed in herds ranging in size from very small (50-sow) to
large (1200-sow) operations. PMWS most commonly affects pigs 2-3 weeks postweaning,
or at about 5-6 weeks of age, although this depends somewhat on the characteristics
of pig flow within the unit. Drafts, overcrowding, poor air quality, and
commingled ages exacerbate the expression and severity of the disease. Based
on our experience, areas of stress and commingled ages exacerbate the expression
and perhaps the severity of the disease. On affected farms, the morbidity
and mortality associated with PMWS appears to be dependent on the degree
of compartmentalization within the unit and the stage of the outbreak. In
an acute outbreak, the mortality rate associated with PMWS may peak at close
to 10% (calculated monthly), whereas endemically infected herds may experience
considerably less morbidity and mortality.

Diagnostic approach

Clinical findings

Suspect herds should be examined thoroughly, noting the age and number
of pigs with characteristic clinical signs (Table 1). Clinical signs, including wasting, dyspnea, and icterus,
are particularly suggestive. With the exception of eperythrozoonosis, which
is extremely rare in Canada and much of the United States, the development
of jaundice in more than a sporadic pig is unusual. Furthermore, the presence
of icterus differentiates the syndrome from PRRS. The most common and consistent
clinical signs of PMWS are wasting and dyspnea.

Postmortem findings

Postmortem examination is absolutely essential for PMWS diagnosis at
the herd level. Because the lesions characteristic of PMWS vary somewhat,
postmortems on several carcasses or euthanized pigs will be required before
the full range of lesions is seen. Selecting the appropriate number or type
of carcass is usually not a problem, as there is normally an over-abundance
of suspect pigs in need of humane euthanasia, especially during acute outbreaks
of the disease. Our recommendation is to keep cutting. A group of 8-12 affected
pigs will normally support the diverse range of lesions that can be noted
grossly. Also unique to PMWS is that typical gross lesions are not always
consistent. For instance, kidneys may appear waxy and enlarged, or spotted
and of normal size. Lungs can be either non-collapsed or atelectatic, and
mottled red or grey (Table 2).

Gross pathology is extremely valuable in ruling out other equally important
bacterial and viral diseases, which can cause the wasting of postweaned
pigs (Table 3).

Histopathologic findings

Histopathology is required for definitive diagnosis. Although not pathognomonic
of the disease, many of the lesions are strongly suggestive (Table 4). Veterinary pathologists who have adequate
experience with the disease are very adept at recognizing characteristic
lesions if a suitable number of representative tissues are submitted (Table 5), regardless of whether
they show gross lesions. Submitting an assortment of live pigs with clinical
disease is strongly recommended during early investigations. These pigs
provide the pathologist with valuable clinical information which is not
normally received with "bits and pieces" submissions. But when
the latter is the only alternative, diagnosis can be made on fixed tissue
submission only. Fresh tissue for bacterial and/or viral culture may assist
in determining the presence or absence of secondary opportunistic or obligate
pathogens.

Unique to PMWS is the presence of intensely basophilic staining inclusion
bodies on H & E sections. The intracytoplasmic inclusion bodies can
be noted in most tissues, but are most prominent in the lymph nodes, tonsils,
and Peyer's patches of the ileum. The inclusion bodies appear to be clusters
of porcine circovirus, according to EM and other tests.

Adjunct diagnostic tests

The lesions of PMWS are associated with porcine circovirus (PCV) infection
based on immunoperoxidase (IP) staining.3 However, at the time
of writing, there is no definitive answer regarding the exact role of PCV
in the pathogenesis of PMWS. Circoviruses are known to cause other nonmammalian
diseases including psittacine beak and feather disease and chicken anemia.4
Recently, a diagnostic polymerase chain reaction has been reported,5
but until the definitive etiology of PMWS is elucidated and Koch's postulates
are fulfilled positive IP and PCR results must be interpreted with caution.
Furthermore, serologic diagnosis will not be possible until an etiologic
agent is identified. It is not yet known whether PCV serologic tests6,7
currently available are useful for the diagnosis of PMWS at the farm level.